Old thread revival, but with regards to twin turbos:
Most twin turbos are like what people have said, two smaller turbos to provide quicker spin up. They can be replaced with one mahooosive turbo to get higher top end power and higher speed but you forfeit acceleration as the bigger turbo takes an aaaaage to spin up.
This isn't quite right. remember if you use two smaller turbos then they only see half the exhaust gasses, so you find that lag an boost threshold will be the same (there are a lot of assumptions I have made here about power output and turbo efficiency being the same etc.)
Twin turbo setups like the Subaru Legacy use two turbos for different parts of the rev range, where you have twin turbos that are the same you tend to find that a single one of a suitable size is not available or the packaging is easier with two; V6 engines for example.
And yes GT17s are a great turbo, but not actually big.
